1. Field of the Invention
The present invention is generally directed to a high frequency heating system of an inverter power supply type which is intended to reduce the weight of a transformer by effecting the conversion into an alternate current having a higher frequency than that of a commercial power supply. More particularly,the present invention is directed to a high frequency heating system designed to prevent generation of abnormally high voltages for a short period of time before initiating oscillations of a magnetron.
2. Description of the Prior Art
A widely utilized heating system as a domestic cooking machine is a high frequency heating system designed to perform dielectric heating by causing a magnetron to generate microwaves and using outputs of the microwaves. In this type of application, it is of importance to reduce both the weight and size of the system. Therefore, in recent years there has been a tendency to employ an inverter power supply capable of decreasing the step-up transformer size and weight by effecting a conversion into an alternate current having a higher frequency than that of the commercial power supply. In the inverter power supply, if a closing period of a switching element increases, a voltage impressed on an anode of the magnetron rises to increase the output. An opening period of the switching element is determined by a circuit constant -i.e., a value which is substantially constant. Hence, a heating output can be controlled by adjusting the length of the closing period by means of a control circuit for controlling the opening and closing operations of the switching element. Thus, the inverter power supply can be reduced in weight and in size, and the heating output thereof can also be controlled by a relatively easy operation.
A variety of feedback control operations have been performed to stabilize the outputs of the inverter power supply. Turning to FIG. 1, there is illustrated a system for effecting the feedback control by detecting a DC input current value. Referring again to FIG. 1, the numeral 1 designates a rectifier circuit; 2 a step-up transformer; 3 a switching element; 4 a capacitor for resonation; 5 a magnetron; 6 a high voltage capacitor; 7 a diode; 8 a detection probe for detecting a primary current; 20 an output control circuit; and 21 a feedback circuit. FIG. 2 shows a system for carrying out the feedback control by detecting a secondary magnetron anode current of a transformer. In FIG. 2, the same components as those of FIG. 1 are marked with the like numerals. Indicated at 9 is a detection probe for detecting a secondary current. Disclosed in Japanese Utility Model Laid-Open No. 62-107397 is a circuit, depicted in FIG. 3, for feeding back a signal obtained on the secondary side of the magnetron step-up transformer to a control circuit for controlling opening/closing operations of the switching element 3 of an inverter circuit. This circuit is fundamentally based on the same principle as that of the system shown in FIG. 2. Referring to FIG. 3, the same components as those of FIG. 1 are marked with the like numerals. The numeral 10 represents a voltage probe for detecting a secondary voltage.
There arise, however, the following problems inherent in the conventional inverter power supplies based on the systems illustrated in FIGS. 1 to 3. After the magnetron 5 has initiated oscillations, the feedback is normally effected. Before starting the oscillations by the magnetron 5 after starting the power supply, however, supply of electric power, i.e., an electric current is not started, though a high voltage is generated on the secondary side of the step-up transformer 2. Based on the prior art systems discussed above, the feedback circuit 2, though a voltage higher than needed is produced, does not function to control the voltage. For this reason, voltages are generated which are twice or three times as high as the voltage required for the magnetron in the conventional circuits. Consequently, in the prior art systems the components--the magnetron 5, the high voltage transformer 2, the capacitor 6 on the side of the high voltage circuit and the diode 7--have to be designed to exhibit properties resistant to abnormally high voltages as compared with voltages during high frequency heating output.
After starting the power supply, a cathode of the magnetron 5 gradually increases in temperature. After a short time, e.g., 3 sec., has passed, electrons are discharged, at which time the abnormally high anode voltages described above are applied. This hinders the normal oscillations by the magnetron 5, and in some cases an overcurrent flows instantaneously. The abnormally large instantaneous pulse current in turn causes an excessive surge voltage in the high voltage circuit or in the switching circuit. As a result, there exists the probability that the switching elements 3, the high voltage diode 7, the high voltage transformer 2 and further the magnetron 5 will be damaged.